ENH un-confuse pos_label use for label indicator matrices #1992
Description
Some metrics take a pos_label argument and interpret it as indicating the positive class in a label indicator matrix (multilabel target representation). This meaning should be removed because it is unusual for pos_label to not be 1, and it can be confused with the positive class label (which corresponds to a column in a label indicator matrix, not a value).
Should pos_label also be removed from LabelBinarizer (where it means "positive indicator value") and fixed to 1? I assume neg_label should not be fixed to 0, as some classifiers work with -1 (however, perhaps that's not a desirable flexibility in label indicator matrices).
I don't particularly like pos_label being used there either, given that label is synonymous with class elsewhere. Perhaps the LabelBinarizer parameters, where not removed, should be renamed to pos_indicator and neg_indicator.
This was brought up at #1983 and #1985 (I accidentally posed it at one when it was meant for the other!).